A good CNG average has many components.
The timing and the spark has to be right, with the right Air/Fuel mixture.
First of all, since you changed your spark plugs, make sure they have the right gap.....
Secondly, your tappet settings should be a bit lose than the petrol setting, and timing should be advanced by around 12 degrees.
An easy way to set the Gas flow valve (the one on the LEFT arrow in your pic) is to hold the race at approx 2500 rpm and reduce the gas flow (turn clockwise) till the engine starts to suffocate....now turn the other way and you will see the engine drowning again in the excess of gas. rotate to clockwise again till the engine is smooth (remember that the race is too be kept still during this operation).
set the idle screw (its on the side of the kit, a bit low; not visible in your pic)so that the car is starting with minimal self load and not missing on idling.
Best of Luck!